You don't have to trust! 0x Exchanges are non-custodial -- you hold onto your funds and atomically execute trades through the 0x smart contracts, which are open source and audited.
Or they can verify them themselves since it's all open source. And for the less technically inclined there's also 3rd party audits and an active bug bounty. Point being, everything is open and transparent, and none of it happens behind the scenes, so that's where the "trustless" comes in.
While yes, that word is overused and misunderstood, and regular users still admittedly do put trust in open source code on a public network, it's a much more open system than we've been able to have before. The trust users depend on in this instance doesn't rely on faith in any single entity to tell you what's changed in their private DB behind the scenes, you can verify it for yourself.
Remember that the smart contract is committed to the blockchain and can never be changed.
Just like when you push your code to a git repo. It's guaranteed immutable, tamper proof. Data side is handled by appending to the blockchain as well. But this doesn't mean it's bug-free.
You have essentially a tamper proof Database with Stored Procedures that doesn't need DBAs. I imagine it as a giant growing BitTorrent file that is maintained (and rewarded) by many computers worldwide. Any tampering to this file, it's immediately detected via hashing (just like in BitTorrent).
The only way to break this is by branching out like you do in git. But in Blockchain, you will have to convince those computers to maintain your fork. And on this note, a Blockchain is technically not controlled by 1 entity, it is the choice of every individual blockchain maintainers (aka miners).
Yes, the thought leaders in the eth community are indeed influential just like Linus Torvalds is to the Kernel project. But that doesn't mean you can't fork Linux should you disagree with him.
What you are saying is not a technical problem. But a problem in Human Society. Or even just an inherent due to it's novelty. This issue is present to almost anything that is not yet widely adapted.
With that said, giant evil mega corporations have grouped up to work on an 'enterprisy' version of Ethereum with a standard spec for interop. We're on a good track to get more 'thought leaders'.
Whether it's a technical problem or a human one, the bottom line is that "guaranteed immutable, tamper proof" is not true. It doesn't matter that Linus Torvalds is influential on the linux kernel project because the linux kernel is not claiming that it is "immutable and tamper proof".
https://www.youtube.com/watch?v=WSxphhWcLxk